How to mix more than two shaders?
The mix shader only directly supports two inputs, as you've seen. But the easiest fix here is just to use two mix shaders:




Mix Shader 1 takes Diffuse and Glossy as input.
Mix Shader 2 takes Mix Shader 1 and another shader as input.


If you want to make the shaders evenly distributed, you could set Mix Shader 2's Fac value to 0.333 (assuming Mix Shader 1 is in the first slot) to give each shader one-third of the overall influence.